Combustion is the rapid reaction of a fuel with oxygen. For economic reasons, air is the source of oxygen in most combustion reactors. The molecular composition of air includes nitrogen, oxygen, argon, and carbon dioxide, and primarily oxygen and nitrogen. Most fuels used in combustion furnaces, such as power plants, are either coal, fuel oil, or gaseous fuels. Such fuels contain carbon and hydrogen elements in their molecular structure. When a fuel is burned, the carbon in the fuel reacts with oxygen to form either CO.sub.2 or CO, and the hydrogen in the fuel reacts with oxygen to form H.sub.2 O. However, at higher temperatures at least a portion of the nitrogen in air reacts to form certain levels of nitrogen oxide (NO.sub.x). Such oxides of nitrogen constitute a major source of air pollution.
Air pollution, while not necessarily lethal, is damaging to human and animal tissue, such as lung tissue. Consequently, multiple limitations on the NO.sub.x content of stack gases vented to the atmosphere as a result of fuel burning have been or are likely to be imposed by various governmental authorities and agencies. One method of reducing the emissions of NO.sub.x includes an oscillating combustion furnace.
Oscillating combustion is the forced oscillation of combustants, such as fuel and/or oxygen, supplied to a burner to create successive fuel rich and fuel lean zones within the combustion chamber of a burner, as taught by U.S. Pat. No. 4,846,665, issued to Abbasi. By oscillating the combustion between fuel rich and fuel lean zones, the fuel burn within the furnace approaches, on average, near stoichiometric, or molecularly complete, burning conditions. However, by oscillating the combustion, the high temperature of stoichiometric burning is avoided, thereby minimizing emissions of NO.sub.x.
In the past, oscillation of the combustants into the combustion chamber has been regulated by a variety of control valves. Such valves regulate the flow rate of the combustibles into the combustion chamber, such that the mass flow rate of the reactants is at a full flow level for a predetermined amount of time and then rapidly ramps down to a low flow mass flow rate. The low flow mass flow rate through the control valve is maintained for a predetermined period until the valve is opened, thereby causing the mass flow to ramp up to the full flow position. Rapidity of ramp down and ramp up is significant to NO.sub.x reduction. Thus, by oscillating the combustants into a combustion chamber of a oscillating combustion furnace, the combustion therein approaches the near stoichiometric proportions and, therefore, delivers the full heating capability, while at the same time minimizing the emissions of airborne pollutants.
Control valves currently available for oscillating burners include mechanical and piezoelectric valves. Poppet type mechanical valves are limited in durability, as well as ability to vary flow between two levels with very small response time. Piezoelectric valves can respond quickly and vary flow between two values, but may also be limited in durability at high temperatures associated with furnaces, due to their use of a flexing elastomeric valve element. Piezoelectric valves have a valve housing, an inlet, an outlet, a piezoelectric actuator assembly, and an elastomeric valve element. The piezoelectric actuator assembly and plunger are disposed within the valve housing, such that the mass flow from the inlet through the valve and out the outlet may be controlled by actuating the piezoelectric oscillating assembly. The piezoelectric oscillating assembly reciprocates the plunger therein to selectively open and close the valve, thereby oscillating flow therethrough. As is well-known, piezoelectric materials may either react to a mechanical stress by producing an electrical charge, or react to an electrical charge by producing a mechanical strain. Current piezoelectric oscillating control valves actuate the piezoelectric oscillating assembly by an electrical charge to produce the desired oscillation of fuel flow into the combustion chamber. However, the useful life of the valve may be shortened because of the adverse effects of the operating temperatures of the furnace on the elastomeric valve element. Therefore, although piezoelectric oscillating assemblies are effective at providing pulse flow into the combustion chamber of a burner, they tend to be sensitive to higher temperatures.
Another form of mechanical oscillating valves utilizes a pair of coaxially aligned aperture plates. The aperture plates are rotatably synchronized with each other to pulse the flow of combustants into the combustion chamber. The aperture plates of such valves have a plurality of holes drilled through each plate. As each plate is rotated, usually in opposite directions of each other, combustants flow through the valve only when the apertures of one plate align with the apertures of the second plate because only at alignment does a passage exist through the valve. Such valves are usually fairly complex because they require more complex mechanical mechanisms to rotate two plates relative to each other.
Thus, there exists a need for a relatively simple and inexpensive control valve for an oscillating combustion furnace that provides controlled flow of combustants into the combustion chamber to minimize the production of NO.sub.x gases. The present invention is directed to fulfilling this need.